Penfieldite

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Penfieldite
Penfieldite-205451.jpg
Perfectly grown, white penfieldite double-ended from the Margarita Mine near Caracoles, Sierra Gorda, Tocopilla, Antofagasta, Chile (image width 2.5 mm)
General and classification
chemical formula Pb 2 Cl 3 (OH)
Mineral class
(and possibly department) 		Halides
System no. to Strunz
and to Dana 		3.DC.15 ( 8th edition : III / D.10)
04/10/01/01
Crystallographic Data
Crystal system hexagonal
Crystal class ; symbol trigonal-dipyramidal; 6th
Room group (no.) P 6 (No. 174)
Lattice parameters a  = 11.393  Å ; c  = 4.024 Å
Formula units Z  = 36
Twinning Contact twins over {21 3 0} to {0001} and {41 5 4}
Physical Properties
Mohs hardness 3 to 4
Density (g / cm 3 ) 5.82 to 6.61
Cleavage clearly after {0001}
colour colorless, white, yellowish, bluish
Line color White
transparency transparent
shine Diamond shine to greasy shine
Crystal optics
Refractive indices n ω  = 2.130
n ε  = 2.210
Birefringence δ = 0.080
Optical character uniaxial positive

Penfieldite is a rare mineral from the mineral class of halides . It crystallizes in the hexagonal crystal system with the chemical composition Pb 2 Cl 3 (OH) and usually only develops small, striped crystals up to about 3 cm in length and prismatic, pyramidal shape.

Etymology and history

Penfieldite was first found in 1892 at various cinder sites in the area around Laurion in the Greek region of Attica and described by Friedrich August Genth , who named the mineral after Samuel Lewis Penfield (1856-1906), a US mineralogist.

classification

In the old (8th edition) and new systematics of minerals according to Strunz (9th edition) , penfieldite belongs to the division of oxyhalides (and related double halides). The new Strunz'sche mineral system, however, now subdivides more precisely according to the cations involved in the formula and the mineral is accordingly in the sub-section “With Pb (As, Sb, Bi) without Cu”.

The systematics of minerals according to Dana also assigns penfieldite to oxyhalides, but according to the classification according to chemical composition in the subdivision "Oxyhalides and hydroxyhalides with the formula A 2 (O, OH) X q " .

Crystal structure

Penfieldite crystallizes hexagonally in space group P 6, space group no. 174 with the lattice parameters a  = 11.393  Å and c  = 4.024 Å and 36 formula units per unit cell .

properties

Pure penfieldite is colorless. However, it can be white, yellowish or bluish in color due to lattice construction defects or foreign admixtures.

Penfieldit dissolves in water and turns it yellowish-white through the formation of lead oxychloride ( Kasseler yellow ).

Education and Locations

Penfieldite, Boleit and Cotunnite from Sierra Gorda, Chile

Similar to laurionite, penfieldite is formed as a secondary conversion product in ancient, lead-containing slag resulting from metal processing under the influence of salt water or in the oxidation zone of lead-containing mineral deposits . There it occurs in paragenesis with cotunnite , fiedlerite , paralaurionite and phosgenite, among others .

So far, the mineral has been proven at 10 sites worldwide: In the “Margarita Mine” near Caracoles in the Chilean community of Sierra Gorda ; in several regions around Laurion in Greece; in the Italian regions of Varenna and Tuscany ; at Mahdia in Tunisia and Mullan in the US state of Idaho .

See also

literature

Web links

Commons : Penfieldite  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d Handbook of Mineralogy - Penfieldite (English, 68.6; PDF; 70 kB)
  2. ↑ a b American Mineralogist Crystal Structure Database - Penfieldite (English, 1995)
  3. ↑ Stefan Weiß: The large Lapis mineral directory . 5th edition, Christian Weise Verlag, Munich 2008, ISBN 3-921656-17-6 .
  4. ↑ a b Penfieldite at mindat.org (engl.)
  5. ↑ American Mineralogist Crystal Structure Database - Penfieldite (English, 1995)